1. Field of the Invention
The present invention relates to a one-component developer and an image forming method, and more specifically to a developer and an image forming method used in a one-component image forming device where a thin developer layer is formed on a developer holder, the layered developer is conveyed to a developing area, and a latent image on an electrostatic latent image retainer is developed with the developer.
2. Description of the Related Art
Currently, in the field of electrophotography dry developing methods, not only electrostatic process copying machines but also printers, facsimile machines and multifunctional integrated machines which comprise a copying machine, a printer and a facsimile are being used. In particular, a developing method capable of achieving downsizing and light weight of a device, conserving resources, and facilitating recycling is increasingly called for from the ecological point of view. Therefore, in order to meet such needs, an image forming method and a developer used therefor are being improved, or a new image forming method and a developer used therefor are being developed. As electro photography dry developing methods in practical use, two-component developing methods where a toner and a carrier such as iron powder are used, and one-component developing methods where a one-component developer not containing a carrier is used are known.
Two-component developing methods are most widely used. However, these methods have disadvantages such as the deterioration of the developer, inability to maintain image quality over a long period of time due to the adhesion of toner particles onto the carrier surface, and an increase in the size of the developing device due to the need for a toner density control system which can stably maintain toner density in the developer and the need for a mixing device for mixing the developer and the toner newly added thereto. Accordingly, one-component developing methods capable of achieving downsizing and light weight of a developing device without requiring a toner density control system are now becoming the mainstream.
One-component toner developing methods can be classified into magnetic one-component developing methods where a magnetic toner is used and nonmagnetic one-component developing methods where a nonmagnetic toner is used. The magnetic one-component developing method is a method where an electrostatic latent image is developed by holding a magnetic toner on a developer holder with a built-in magnetic field generation means such as a magnet, adjusting the thickness of the magnetic toner layer with an elastic blade formed from silicone rubber, and adhering the toner to a latent image on a latent image retainer. Since the control of toner transport is easy and dirtying of a device such as a copying machine or a printer is minimal, this method currently is in wide practical use. However, because the magnetic toner used in the magnetic one-component developing method contains a black- or brown-colored magnetic substance such as magnetite, the method has the disadvantage of the inability to form full-color images, which is now increasingly required in the market.
On the other hand, a nonmagnetic one-component developing method is a method in which a latent image is developed by supplying a nonmagnetic toner to a developer holder via a toner supply roller and an agitator which are adjacent to the developer holder, adjusting the thickness of the toner layer with a layer regulating blade, and adhering the toner to a latent image on a latent image retainer. Since the toner does not contain a colored magnetic substance, color images can be formed using this method. Further, since the developer holder does not comprise a magnet, the device can be made more light weight and compact and the cost of the device can be reduced, and thus this method is now being applied in compact full-color printers.
Unlike the two-component developing method, the one-component developing method does not comprise a carrier, which is a stable charging and transport means for the toner. Therefore, characteristics required for a toner used in the one-component developing method are more strict than that required for a toner used in the two-component developing method. In particular, in the case of a nonmagnetic one-component developing method, since magnetic force is not utilized for toner layer formation or toner transport, the toner must be held on the developer holder mainly by the force of static electricity. Accordingly, a toner used in the nonmagnetic one-component developing method is required to have a characteristic allowing it to be charged quickly and evenly and a characteristic allowing it to have good flow for stable layer formation. In a case in which these characteristics are not sufficient in the toner, there is a risk of causing image density deterioration, fogging, developing ghosts, dirtying of the device over the long term, and the like.
In addition to these characteristics of developing, other characteristics are required of a toner for achieving a compact light-weight device, improving image quality, and solving ecological problems. For example, a contact-type bias roller is used predominantly for charging a latent image retainer or for transferring the toner in place of a conventional corotron in order to prevent the generation of ozone. Accordingly, high transfer efficiency and good cleaning properties are required for the toner to prevent adhesion to the charging roller. In addition, for preventing dropout caused by a contact-type transfer roller, good adherence and aggregation properties and flow are required.
Conventionally, fine powders of an inorganic oxide are added for stabilizing the charge and transport of the toner by improving the flow and charge property of the toner. Fine silica powders are commonly used as an inorganic oxide because they are characterized by being capable of dramatically improve toner flow. However, since fine silica powders have a strong negative charge, they are not appropriate for a positively charged toner even if they are treated with a compound containing nitrogen for decreasing the negative charge. Further, since silica maintains a strong negative charge in a low-temperature and low-humidity environment and the charge property thereof is lowered in a high-temperature and high-humidity environment due to moisture absorption, thereby the charge property of a toner to which silica is added drastically fluctuates due to environmental change. As a consequence, it is not possible to achieve optimal transporting of the toner onto the developer holder nor an optimal charge property of the toner in both a high-temperature and high-humidity environment and a low-temperature and low-humidity environment, and thus there are problems such as poor image density, background fogging, and dirtying of the device.
In order to solve these problems, a method of treating the surface of inorganic fine powder to be added to a toner is proposed. For example, methods of a hydrophobic surface treatment of fine silica particles are disclosed in Japanese Patent Application Laid-Open (JP-A) Nos. 46-5782, 48-47345, 48-47346, 59-34539, 59-198470, and 59-231550. However, treated fine inorganic powders are insufficient for preventing fluctuation of the charge property.
Furthermore, as method of decreasing the negative charge of silica, a method of treating the surface of fine silica particles to be added to the toner surface with an amino-modified silicone oil (Japanese Patent Application Laid-Open (JP-A) No. 64-73354), and a method of treating the surface of fine silica particles with amino silane and/or amino-modified silicone oil (Japanese Patent Application Laid-Open (JP-A) No. 1-237561) are known. Such methods of treatment with an amino compound can curb the excessive charge increase of a negatively charged toner in a low-temperature and low-humidity environment, but cannot sufficiently prevent fluctuation of the charge property of fine silica powders due to environmental change.
Furthermore, a toner with silica added in an amount sufficient for providing sufficient flow cannot achieve the rapid charge speed required for a nonmagnetic one-component developer even when the average value of the charge amount of the toner is in a preferable range, because of the high resistance, the high charge property and the low charge speed of fine silica particles. Therefore, toners on a developer holder have a wide charge distribution so that fogging and toner clouding (dirtying of the device) are caused.
As mentioned above, a method involving hydrophobic treatment of silica or a method of decreasing the negative charge cannot sufficiently improve fluctuation of the charge property due to environmental change, a slow charge speed, and a wide charge distribution.
On the other hand, for preventing the dropout caused by a contact-type transfer roller, a method of treating fine silica powders added to the toner surface with a silicone oil is advocated. According to this method, dropout can be prevented but the wide charge distribution of the toner cannot be improved, and thus it is difficult to prevent fogging, toner clouding, and the like.
In order to improve the charge property and the flow of a toner, titania is also commonly used as an inorganic oxide to be added to a toner. Titania has a faster charge speed and low resistance compared to silica, and thus the charge distribution curve of a toner with titania added is sharp. In addition, titania curbs the charge increase of a developer and ghost phenomena. However, titania cannot provide a high charge to a toner, and thus deterioration of the amount of toner transported, deterioration of the density reproduction property caused by the decreased charge, background fogging, and dirtying of the device are easily generated.
For improving the charge property of titania, methods of the hydrophobic treatment of titania added to the toner surface are proposed (Japanese Patent ApplicationLaid-Open (JP-A) Nos. 58-216252, 60-123862, and 60-238847). Hydrophobic titanium oxides can be obtained by treating the surface of titanium oxide with a silane compound, a silane coupling agent, a silicone oil, or the like. With this method, the charge property of the toner can be improved more in a case in which an appropriate type and amount of a processing agent is selected, rather than in a case in which untreated hydrophilic titania is used as the external additive. However, the charge property of a one-component toner cannot be improved to a preferable degree. In addition, as to reducing charge property fluctuation with environmental change, the method has its limitations. In addition, titanium oxide with a process agent added is drastically inferior to a hydrophilic titanium oxide in terms of the charge speed, the sharpness of the charge distribution curve, and the like.
Moreover, titanium oxide is obtained by a sulfuric acid method or a hydrochloric acid method, which is a wet method. In these methods, mainly using an ilmenite ore as the material, titanium oxide is obtained by generating titanium hydroxide from the ilmenite ore and baking the generated titanium hydroxide. However, in a sulfuric acid method or a hydrochloric acid method, aggregated (secondary or tertiary aggregation) particles are generated as a consequence of dehydration condensation, and it is not easy to disperse such aggregated particles again with conventional technology. Therefore, the effect of improving the flow of a toner of titanium oxide obtained in a sulfuric acid method or a hydrochloric acid method is significantly inferior to that of silica. In order to achieve a high image quality, which is increasingly required by the market, attempts have been made to obtain finer toner particles, but by having finer toner particles, the adhering force among particles is strengthened, resulting in further deterioration of the flow of the toner. In the case that titanium oxide produced in a sulfuric acid method or a hydrochloric acid method is used in such toner particles, deterioration of the toner flow is particularly marked. In addition, the specific gravity of titanium oxide conventionally used is higher than that of silica, and thus it has problems such as the inability to be bonded firmly onto the toner surface and the tendency to peel easily away from the toner surface. Accordingly, titanium oxide has a poor long-term charge stability, and, in addition, it can easily cause sleeve dirtying and dirtying of a latent image retainer which results in image quality deterioration and image defects.
Accordingly, in order to achieve both improvement of the flow and prevention of the fluctuation of charge property with environmental change, a method of adding both hydrophobic titanium oxide and hydrophobic silica to a toner is disclosed (Japanese Patent Application Laid-Open (JP-A) No. 60-136755). In this method, it is difficult to uniformly disperse the additives on the toner surface during use, and thus the dispersion state changes due to the stress on the sleeve so that the influence of either the hydrophobic silica or the hydrophobic titanium oxide is easily observable. Therefore, it is difficult to stably cover the shortcomings of both additives over the long term.
Furthermore, in a process of generating titanium oxide in a wet method, a method of hydrolyzing a silane compound in a water-containing media, treating the surface of titanium oxide with the hydrolyzed product of the silane compound, producing titanium oxide particles in a state where aggregation is retarded, and adding particles in a toner is proposed (Japanese Patent Application Laid-Open (JP-A) No. 5-188633). Such treatment with a silane compound improved the toner flow with fewer aggregated particles compared to a conventional hydrophobic treatment method of titanium oxide, but cannot improve the charge property of a negatively charged toner, cannot prevent the fluctuation of the charge property due to environmental change, and exerts and adverse influence in terms of the charge speed (admix property of added toner) and charge distribution.
Furthermore, a method of adding hydrophobic amorphous titanium oxide to a toner is proposed (Japanese Patent Application Laid-Open (JP-A) Nos. 5-204183 and 5-72797). Amorphous titanium oxide can be obtained by hydrolyzing metal alkoxide or metal halide by the CVD method (Chemical Engineering Articles (Vol. 18, No. 3, 303-307 (1992)). Titanium oxide obtained by the hydrolysis method improved the charge property and the toner flow but because it has much adsorbed water within particles, it is liable to remain on the latent image retainer at the time of toner transfer, resulting in dropout on the image or damage on the latent image retainer at the time of cleaning.
In addition, with the toner having an inorganic oxide added to the surface thereof, due to the stress applied to the toner in long-term use, filming or adhesion is generated, the external additive peels off of the toner particles, or the external additive is buried in the toner particles, and thus the toner charge property changes. Accordingly, it is difficult to control the charge and transport of the toner over the long term.
In order to solve these problems, a method of using a specific binder rein for preventing the external additive from being buried is proposed in Japanese Patent Application Laid-Open (JP-A) Nos. 6-95429, 6-102699, and 6-266156. A method of using a specific charge control agent and an external additive is advocated in Japanese Patent Application Laid-Open (JP-A) Nos. 6-51561, 6-208242, and 6-250442. However, neither of these methods can completely solve the above-mentioned problems. In particular, in a full-color developing system, in which control of the amount of toner used in developing a latent image accurately is needed, there are problems still remaining in terms of stabilizing the amount of toner charge and toner transport over the long term.